1. Field of the Invention
The invention relates to a switching bridge amplifier comprising: an amplifier stage having an input for receiving an input signal to be amplified, and having an output for presenting an output voltage which varies in a positive and a negative sense relative to a reference voltage, a reference voltage terminal for connecting a reference voltage source for a reference voltage supply, first and second load terminals for connecting a load, the first load terminal being coupled to the output of the amplifier stage, a plurality of at least one positive switch voltage terminals, for connecting respective positive switch voltage sources to respective switch voltages which, in ascending steps deviate in a positive sense from the reference voltage, a plurality of at least one negative switch voltage terminals, for connecting respective negative switch voltage sources to respective switch voltages which in ascending steps deviate in a negative sense from the reference voltage, switch means comprising a reference switch, a number of positive switches corresponding to the plurality of positive switch voltage terminals, and a number of negative switches corresponding to the plurality of negative switch voltage terminals for connecting the second load terminal to the reference voltage terminal, one of the positive switch voltage terminals or one of the negative switch voltage terminals, and control means for generating control signals to activate the reference switch if the input voltage differs less than a threshold from the reference voltage, and to activate always one of the positive or negative switches respectively, if the input voltage differs more than the threshold from the reference voltage.
2. Description of the Related Art
A switching bridge amplifier of this type is known from European Patent application EP-A 0 147 306. In the conventional linear bridge amplifier, the second load terminal of the load is connected to a second amplifier stage identical with the first amplifier stage, which second amplifier stage is driven in opposite phase relative to the first amplifier stage. Such a configuration is known as a bridge amplifier and has for its object to generate with a given supply voltage a larger power in the load than is possible with a single amplifier stage. Without a driving input signal, the output voltages of the amplifier stages are equal to the reference voltage. With an increasing input signal the output voltages will increase in a mutually reverse sense. A positive voltage excursion relative to the reference voltage at the output of the first amplifier stage is accompanied by a negative excursion relative to the reference voltage at the output of the second amplifier stage, and vice versa. Thus, a peak-to-peak voltage may appear across the load, which voltage is twice as large as in a single amplifier stage in single-ended configuration, in which configuration one of the load terminals is connected to a fixed reference voltage. This may quadruple the power generated in the load.
Bridge amplifier circuits of this type are often used in audio signal amplifiers in which the load is a loudspeaker. More specifically, in battery-operated amplifiers having relatively low available supply voltages, such as in portable radio sets and car radios, the bridge circuit is used.
A disadvantage of the linear bridge amplifier circuit is that the mean power dissipation in the amplifier stages is relatively high. More specifically, in the event of relatively little power generated in the load, the heat dissipation in the amplifier stages is relatively high. The relatively small current through the load flows to the supply via the amplifier stages across which there is a relatively large voltage drop at that moment. This situation, which leads to a relatively large heat dissipation in the amplifier stages, statistically often occurs with audio signals.
From aforesaid European Patent application, it appears that a single amplifier stage in single-ended configuration will suffice, provided that for reaching the limit of th eoutput swing of this amplifier stage, the second load terminal normally connected to the reference voltage is connected to a switch voltage which is positive or negative relative to the reference voltage and the attendant voltage jump across the load is compensated by a suitable negative feedback of the voltage jump to the input of the amplifier stage. The operating point of the amplifier stage output then shifts as much as the voltage jump on the second load terminal. This provides room for a still larger output swing at the output of the amplifier stage. One half of this known bridge amplifier circuit is then not arranged as a linear amplifier, but as a switch which connects the load to a number of switch voltages which are positive and negative relative to the reference voltage. With this switched bridge amplifier configuration, a considerable dissipation reduction can be achieved compared with a bridge amplifier circuit comprising two linear amplifier stages which have the same supply voltage.
The known switching bridge amplifier comprises a reference switch which connects substantially without dissipation, the second load terminal to the reference voltage if the input signal, and thus also the output signal of the amplifier stage remains below a specific threshold relative to the reference voltage. The circuit then operates as a normal single-ended amplifier with the attendant dissipation which is lower relative to a standard linear bridge circuit. Furthermore, a plurality of at least one positive and negative switch voltage terminals are provided to which positive and negative switch voltages relative to the reference voltage are fed, and corresponding positive and negative switches which are activated if the input signal exceeds the threshold. Depending on the polarity of the output signal, a positive or negative switch is activated. Alternatively, it is possible to make a one-step change to the most positive or negative available switch voltage once the input signal has exceeded the threshold value. All this takes place under the control of control means which activate the correct switch in dependence on the value of the input and output signals of the amplifier stage.
The reference switch is to be capable of conducting the current through the load to the reference voltage terminal in two directions. Once the input signal has exceeded the threshold, the reference switch is to be rendered non-conductive and one of the positive or negative switches is to be rendered conductive.
The reference switch may be arranged in known manner comprising two transistors of opposite conductivity types connected in anti-parallel while a diode is included in the collector line to avoid reverse bias of the transistors. This configuration causes a slight voltage jump to occur across the reference switch each time the direction of the current through the reference switch is reversed. This voltage jump is fed to the input of the amplifier stage via the negative feedback means, so that this stage generates an equally large voltage jump at the output. For this purpose, the amplifier stage is to be capable of changing the voltage on its output very rapidly. Too small a bandwidth and/or slew rate causes signal distortion.